Guralp Systems CMG-6T - Manual (Page 8)

CMG-6T
2.4 Centring (zeroing) the instrument
For optimal operation, the instrument's
masses should be at their centre
positions in the absence of a seismic
signal. The dynamic range of the
instrument is reduced if the masses
stray too far from the centres. The
mass positions are affected by the angle
at which the instrument is installed as
well as by the ambient temperature.
The instrument should be allowed to
acclimatise itself before final centring
is carried out.
The mass positions are directly output by the instrument and are
available from the bayonet connector where they can be monitored by
a Güralp Systems digitiser (where they appear as streams M8 (vertical),
M9 (N/S) and MA (E/W)) or by using the meter on the hand-held
control unit. The offsets can be nulled using three potentiometers,
located under a protective cover on the instrument's lid and adjustable
using a trim-pot tool or a very small flat-bladed screwdriver.
The cover can be removed without
tools: three potentiometers are then
exposed. The one nearest the
“North” of the instrument controls
the vertical mass offset and the
other two, proceeding clockwise,
control the North/South and East
West offsets.
1. Select the component you want to centre using the CENTRING
SELECT dial;
2. Switch the signal meter dial to one of the MASS POS settings;
3. Switch the rightmost switch to 1 SEC VEL to enable the centring
lines;
4. Switch the RANGE selector to 10V (coarse adjustment);
5. Adjust the relevant potentiometer until the signal meter reads
zero.
6. Switch the RANGE selector to 1V (fine adjustment);
8 Issue B